The present disclosure generally relates to a system and a method for fatigue analysis of a bottom hole assembly.
To obtain hydrocarbons, a drilling tool is driven into the ground surface to create a wellbore through which the hydrocarbons are extracted. Typically, a drill string is suspended within the wellbore. The drill string has a drill bit at a lower end of sections of drill pipe. The drill string extends from the surface to the drill bit. The drill string has a bottom hole assembly (“BHA”) located proximate to the drill bit.
The BHA has drill collars housing one or more tools, such as measuring devices, power supplies, motors, stabilizers or the like. Drill collars provide weight on the drill bit for control and penetration of the drill bit. The drill collars are connected to each other by threaded connections. Port holes in the drill collar may be used for various purposes.
Fatigue is a cause of drill pipe and drill collar failures. Port holes, dimension changes, such as external radii, and threaded connections are susceptible to fatigue-induced failures. Consequently, an accurate estimation of the fatigue life of a weakest component of the BHA is beneficial for proper planning for and reliable completion of drilling operations. In addition, an accurate estimation of the fatigue life enables design changes which enhance fatigue resistance of the weakest component in the BHA. The fatigue life of a component is usually defined as the total number of cycles to initiate a dominant crack and to propagate this dominant flaw to final failure.
The fatigue damage process may be described as the nucleation and growth of cracks to final failure. The evolution of permanent damage under cyclic deformation, and the nucleation and the propagation of a fatigue crack involve the microstructural length scale. However, phenomenological continuum approaches are widely used to characterize the total fatigue life as a function of controlling parameters, such as, for example, the applied stress range, the strain range, the mean stress, and the environment. The stress-based method and the strain-based method address the damage evolution, the crack nucleation and the crack growth stages of fatigue using a single experimentally characterizable continuum formulation.
Fatigue design is usually undertaken based on the similitude concept. The behavior of small-scale material specimens under cyclic test with carefully controlled conditions is related to the likely performance of real structures subjected to variable amplitude fatigue loads under either simulated or actual service conditions. The commonly used approaches for fatigue life prediction include stress life approach, strain life approach and crack growth life approach. The stress life approach is suitable for infinite life or high-cycle finite life applications due to its incapability of capturing notch plasticity. Prediction of the crack growth life using fracture mechanics approaches utilizes accurate determination of the initial crack size which may be difficult. Another limitation of the crack growth life approach is the difficulty in accounting for complex sequence effects. The strain life approach is better suited for fatigue analysis of structures with finite fatigue lives because the strain life approach captures effects due to complicated load sequences.
Nevertheless, fatigue is a complex process. Numerous factors may affect the fatigue life of a structure, such as, for example, cyclic stress states, geometry, surface quality, material type, residual stresses, size and distribution of internal defects, grain size, environment and temperature. Consequently, given the large uncertainties associated with fatigue life predictions, a realistic fatigue analysis approach benefits from using conservative parameters to enable consistent comparative analysis of different components of a BHA.